It was using __GNUC__ < 8 to check if _xgetbv intrinsic is available.
Since we always use zig cc with this header, we know we have clang 9,
which does have this intrinsic.
also start prefering NtDll API. so far:
* NtQueryInformationFile
* NtClose
adds a performance workaround for windows unicode conversion. but that
should probably be removed before merging